A data storage device based on the atomic force microscope (AFM) is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. The probes operate in parallel with each probe scanning, during operation, an associated field of the storage medium. The storage medium comprises a polymethylmethacrylate (PMMA) layer. The tips, which each have a diameter of between 5 nm to 40 nm, are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying forces to the probes so that the tips of the probes can touch the surface of the polymer layer. For this purpose, the probes comprise cantilevers, which carry the tips on their end sections. Bits are represented by indentation marks, each encoding a logical “1”, or non-indentation marks, each encoding a logical “0”, in the polymer layer. The cantilevers respond to these topographic changes while they are moved across the surface of the polymer layer during operation of the device in read/write mode.
Indentation marks are formed on the polymer layer by thermomechanical recording. This is achieved by heating the tip of a respective probe operated in contact mode with respect to the polymer layer. Heating of the tip is achieved via a heater dedicated to the writing/formation of the indentation marks. The polymer layer softens locally where it is contacted by the heated tip. The result is an indentation mark, for example, having a nanoscale diameter comparable to the diameter of the tip that is used in its formation, being produced on the layer.
Reading is also accomplished by a thermomechanical concept. For this purpose a heater dedicated to the process of reading/sensing the indentation marks is energized. The heater is thermally decoupled from the tip of the probe. Accordingly, softening of the polymer layer does not occur where it is contacted by the tip during reading. The thermal sensing is based on the fact that the thermal conductance between the heater and the storage medium changes when the probe is moving in an indentation mark as the heat transport is in this case more efficient. As a consequence of this, the temperature of the cantilever decreases and hence, also its electrical resistance changes. This change of electrical resistance is then measured and serves as the readback signal.
The configuration in which previously-proposed devices are formed is that the data storage medium, usually a polymer layer, is formed on a substrate that has a comparably higher shear modulus. FIGS. 1a and 1b in the accompanying drawings respectively show examples of such previously-proposed data storage devices. In FIG. 1a, a data storage medium 3 comprising a polymer layer is deposited onto a silicon substrate 4. The device of FIG. 1b differs from that of FIG. 1a in that the data storage medium 3 is deposited onto a secondary layer 5 that has a higher shear modulus than the polymer material of the data storage medium 3. The secondary layer 5 may, for example, be SU8 that is provided on the silicon substrate 4. A role of the secondary layer 5 could be seen as preventing penetration of the tip, which is used to produce indentation marks on the polymer layer, through the polymer layer since this would cause contact between the tip and the silicon substrate 4 and, therefore, tip wear.
The geometry of the indentation marks produced in the polymer layer may play a role in determining a data density (the number of bits per unit area) and, therefore, a data storage capacity, of the above-described data storage devices. In particular, when an indentation mark is formed in the polymer layer, polymer material is displaced from where the indentation mark is formed and deposited onto the region surrounding the opening thereof, thereby forming a rim around the indentation mark. Rim formation around indentation marks may limit the data density, this especially being the case when the distance between the bits is reduced to, for example, 20 nm, in order to increase a storage capacity of the data storage device. Furthermore, the rim formation also affects the readback signal obtained during the read operation since the height of the surface between adjacent bits, i.e. where a non-indentation mark is present/logical “0” is encoded, is increased relative to what is obtained for an indentation mark. Thus, the noise associated to the non-indentation marks or logical 0's may be increased, which causes lowering of the signal-to-noise (SNR) ratio.
Accordingly, it is desirable to provide a data storage device in which rim formation around an indentation mark is reduced compared to what is obtained in previously-proposed data storage devices.